In the reproduction of continuous tone information for mechanical printing purposes, it is customary to make a half-tone photographic intermediate, usually a film negative, in which the gradations in tone are represented by dots of differing size. The quality of the resulting halftone picture is closely connected with the shape, spectral density, and, uniformity of the dots of the half-tone print.
In order to obtain very high-contrast screen dots in halftone images it has been practice to formulate developers, so-called lith-developers, containing essentially a p-dihydroxybenzene such as hydroquinone, an alkali, an alkali metal bromide and a low level of free sulfite ions.
Very high contrast results, preferably with gamma above 10, also called "lith-gradation", can be obtained with said high-contrast developers and so-called "lith silver halide emulsion materials". In these materials the silver halide comprises at least 50 mole % of chloride, the balance, if any, being bromide and optionally a minor amount of iodide.
Hydroquinone developers having a low sulfite ion concentration are commonly referred to as "lith-type developers" and their mechanism of operation has been described for the first time by J. A. C. Yule in the Journal of the Franklin Institute, 239 (1945), pages 221 to 230.
The properties of lith-type developers are believed to result from autocatalytic action, often called "infectious development", due to a local high concentration of the oxidation products of the developing agent, which can build up as a result of the low sulfite ion concentration that has to be kept at low level to maintain the lith-development characteristic. This is achieved in all known commercial developers of this type by the use of the addition product of formaldehyde and sodium hydrogen sulfite, i.e. sodium formaldehyde hydrogen sulfite, which acts as a sulfite ion buffer.
Convential "lith" developers suffer from deficiencies which restrict their usefulness. For example, the developer exhibits low development capacity as a result of the fact that it contains hydroquinone as the sole developing agent. Also, the aldehyde tends to react with the hydroquinone to cause undesirable changes in development activity. Furthermore, the low sulfite ion concentration is inadequate to provide effective protection against aerial oxidation. As a result, a conventional "lith" developer is lacking in stability and tends to give erratic results depending on the length of time that it has been exposed to air.
With the advent of processing machines and more particularly, rapid access processing machines, the deficiencies of lithographic processing chemistry became serious and apparent. Developer degradation was accelerated under machine processing conditions. The lag in the start of development caused by the long induction period of the hydroquinone developers lengthened the processing time and delayed access to the finished product.
As explained e.g. in U.S. Pat. No. 4,081,280 a lith-developer with low free sulfite content has to be replenished carefully for compensating for developer exhaustion by aerial oxidation.
By using so-called "rapid-access" developers containing both hydroquinone and an auxiliary developing agent, e.g. a 1-phenyl-3-pyrazolidinone type developing agent or N-methyl-p-aminophenol sulphate, the induction period can be eliminated and the developing process speeded up.
Developer liquids containing a fairly high amount of sulfite and hydroquinone in combination with an auxiliary developing agent are more stable with respect to oxidation by oxygen of the air than developers having a relatively low sulfite content and containing hydroquinone as the sole developing agent. However, the trouble is that rapid access developers containing said auxiliary developing agents are not suited for use in linework or halftone image production because they cannot produce the necessary high gradient. Thus, the combination of combine high contrast development with the processing convenience and stability of the rapid access developers remains a goal in this field.
It is generally known that the sensitometric properties (e.g. speed and/or gradient) of a photographic silver halide emulsion material can be controlled by the silver halide emulsion preparation, e.g. by the silver halide grain size, its distribution and chemical sensitization of the silver halide grains. Sulphur sensitization is the most widely used method of conferring speed and contrast on a silver halide emulsion [ref. Photographic Emulsion Chemistry by G. F. Duffin--The Focal Press--London and New York (1966), p. 84].